The present invention relates to an antenna unit which is mainly used in a small mobile radio set, and more particularly to such an antenna unit that good radiation characteristic can be obtained even when it is used in proximity to a human body.
Recently, a demand for mobile radio sets such as mobile telephones has been sharply growing, and a compact, lightweight and thin radio set has been sought for.
As a conventional antenna for a mobile telephone, a fixed type helical antenna, a planner inverted F antenna, etc. have been popular, and an antenna system of a small size which has good portability and will not cause an inconvenience when it is used with a compact radio set has been realized.
FIG. 5 is a view of an outer appearance of the fixed type helical antenna which has been widely used as the conventional antenna for the mobile telephone. The fixed type helical antenna element 10 is provided on a main body 11 of a mobile telephone set to realize a compact and lightweight antenna system.
However, the fixed type helical antenna as shown in FIG. 5 is so constructed as to protrude from the main body of the radio set in order to obtain desired characteristics. Therefore, it has been a problem that a protruded portion of the antenna will be an obstacle in case where the radio set is carried in a bag or in a pocket, and the antenna tends to be broken when the radio set has fallen. For this reason, in Japanese Publication No. Hei. 8-316725 of unexamined Patent Application, there is proposed a helical antenna which can be incorporated in the main body of the mobile telephone set as shown in FIG. 6. This antenna 90 comprises a helical antenna element 94 and an inductor 91 integrally formed therewith. By incorporating the antenna in the main body of the mobile telephone set, such a design of the mobile telephone set that the antenna element will not protrude from the main body of the mobile telephone set can be realized.
The helical antenna as shown in FIG. 5 or 6 can be generally considered as the antenna which is composed of a loop antenna and a dipole antenna connected in series. The loop antenna is a magnetic current type antenna equivalent to a magnetic current element which is placed at a loop center of a loop face perpendicularly to the loop face, while the dipole antenna is an electric current type antenna. The helical antenna is considered to be equivalent to those antennas combined.
However, since both the fixed type helical antenna in FIG. 5 and the incorporated type helical antenna in FIG. 6 are supplied with electric power through a feeding line of an unbalanced system such as a coaxial line, a large amount of earthed electric current flows not only on the antenna element but also on a base plate of the radio set, and as the result, the antenna usually functions as the electric current type antenna. Therefore, when the radio set is used in proximity to the human body, the antenna will be influenced by the human hands or head resulting in a serious deterioration of gain.
FIG. 7 is a characteristic diagram representing radiation directivity of the conventional fixed type helical antenna. There has been a drawback that as a result of a large amount of the electric current which has flowed not only on the antenna but also on the base plate of the radio set, a xcex8-component, that is, an electric current component is dominant, and the antenna will hardly function as the magnetic current type antenna. As the result, there has been such an inconvenience that in a state where the radio set is carried by the human hand and tiltedly used, polarization of an incoming wave from a base station and polarization of the antenna on the radio set will not match, and accordingly, receiving performance will be seriously deteriorated.
The invention has been made to solve the above described problems in the conventional art, and its object is to provide a compact and high-gain antenna unit which can efficiently receive the incoming wave even though the radio set is used tiltedly.
Therefore, according to the invention, in an antenna nit adapted to be incorporated in a main body of a mobile radio set, there are provided a pair of helical antenna elements connected to a balanced feeding line, the helical antenna elements having antenna axes in a direction parallel to a base plate of the radio set and being arranged in proximity to the base plate of the radio set at a sufficiently small distance as compared with their wavelength.
As the result, by conducting a balanced power supply to the helical antenna elements, an electric current component flowing on the base plate of the radio set can be reduced, thus decreasing the influence of the human body when the radio set is carried by the human hand.
Additionally, the antenna can possess both radiation characteristics of the electric current type antenna and the magnetic current type antenna, and therefore, the receiving performance when the radio set is tiltedly used will be enhanced.
Further, a compact and thin antenna system can be realized.